1. Technical Field
The present invention relates to a duplexer device.
2. Description of Related Art
A duplexer including a surface acoustic wave (SAW) device is mounted on a communication device, such as a handheld terminal, in order to electrically separate the transmission path and reception path of an antenna.
Different frequency bands are allocated to communication devices depending on areas and countries. In order to use the same communication device in several areas and countries, a plurality of duplexers corresponding to many bands needs to be mounted on the communication device. Furthermore, as the number of bands used for communication recently increases, the pass band of a duplexer partially overlaps a frequency band used in another communication system, such as a television signal, in some countries. An example in which the pass band of a duplexer partially overlaps a frequency band used in another communication system is described in more detail below with reference to FIG. 7.
FIG. 7 shows the transmission band 61 and reception band 62 of a duplexer and shows that a band on the low frequency side of the transmission band 61 partially overlaps another band 63 used in another communication system, for example, a television system. In FIG. 7, a horizontal axis is a frequency MHz and a vertical axis is an insertion loss dB.
The transmission band 61 is divided into a block-A including a frequency band overlapping another band 63 and a block-B including a frequency band not overlapping another band 63. A band in which the block-A and the block-B overlap is used as a real transmission band. Likewise, the reception band 62 is divided into a block-A and block-B corresponding to the transmission band 61. A band in which the block-A and the block-B overlap is used as a real reception band.
As shown in FIG. 7, if some of the transmission band 61 overlaps another band 63 and another band 63 is used, a stop band 64 has to be formed in the block-A included in the low frequency region of a real transmission band. Although some of the transmission band 61 overlaps another band 63, it is necessary to handle a case where another band 63 is not used.
For this reason, there is a need to prepare a duplexer designed to stop a stop region and transmit a real transmission band and a real reception band and a duplexer designed to transmit a stop region and also transmit a real transmission band and a real reception band. However, if a plurality of duplexers having different filter characteristics is installed, it becomes an obstacle to a reduction in the size of a handheld terminal. Furthermore, cross wires become complicated because a switch for switching two duplexers is also required.